The present invention relates to a method of forming conductive oxide layers and more particularly the method of forming the anode, cathode, and electrolyte layers in a tubular, planar, or monolithic type solid oxide fuel cell.
One method for the conversion of chemical energy directly to electrical energy is by the use of fuel cells. The three main types of fuel cells now being developed for industrial application in the hundred of megawatts range are phosphoric acid fuel cells, molten carbonate fuel cells, and solid oxide fuel cells. All three types are essentially chemical cells having an anode where the fuel is absorbed, a cathode where oxidants are absorbed and an electrolyte where fuel and oxidant are reacted.
In the solid oxide fuel cell, the electrolyte is normally composed of a thin layer yttria stabilized zirconia, the anode or fuel electrode is a cermet of nickel and zirconia and the cathode or air electrode is a strontium doped lanthanum manganite. The interconnectors are normally magnesium doped lanthanum chromite.
There are three main types of solid oxide fuel cells: the tubular design, the planar design, and the monolithic design. The present invention which relates to the method of forming these composite structures of thin sheets of conductive oxides may be applied to any type of solid oxide fuel cell but will be described with particular references to the monolithic solid oxide fuel cells.
The material integrity of the monolithic solid oxide fuel cell and the intrinsic electronic structure of the triplex layer (anode, cathode, and electrolyte), and the interconnector dominate the working efficiency of the module. Internal cracking, delamination, and bubbles have been the cause of a large number of failures in the internal fabrications of these monolithic oxide stacks. Analyses have shown that the possible cause of failure may be due to the manufacturing process for the removal of binder and sintering of the oxides, due to inherent uneven heating and differential thermal expansion during temperature cycling. One of the greatest inherent problems in the processing of the ceramics is the removal of the binder by decomposition and removal of the gas phase at relatively low temperatures and the lack of a uniform temperature throughout the entire ceramic system. Any temperature differential within the same material and/or different rates of decomposition between dissimilar materials will cause stress related areas within the system these stress areas will be the cause of separation of contact points and/or separation at the interfaces of surfaces during the fabrication process or during operation of the cell.
The present method of heating composite structures of the type used for solid oxide fuel cells is by radiation heating in resistence-type furnaces or in hot gas convection furnaces. With radiation heating, the internal region of the thin layer monolith is heated by thermal conductions. The very design of the structure is an efficient thermal insulation, which means that radiation heat impinging on the outer surface of the structure, will take a considerable time before the internal of the cell reaches the same temperature. Large temperature gradients are, therefore inevitable. During the removal of binder with temperatures on the order of 400.degree. C., the temperature differential within the structure will be proportionally greater than the temperature differential at the sintering temperatures of 1400.degree. C. to 1600.degree. C. During the removal of binder, the variations in temperature and, therefore, the variations in the rate and extent of decomposition of the binders will result in distortion, delamination, and cracking.